Plasma processing is well known as a surface processing method for cleaning or etching an object to be processed, such as a board on which electronic parts are mounted. In the plasma processing, the board to be processed is placed in a vacuum chamber forming a processing chamber. Then, plasma discharge is generated in the processing chamber such that ions and electrons generated as a result of the plasma discharge are made to act on the surface of the board, whereby desired surface processing is executed. In order to stably execute the plasma processing at high quality, it is necessary to generate plasma discharge properly in response to discharging conditions which have been previously determined according to a purpose of processing. Therefore, for the purpose of monitoring a state of the generation of the plasma, various means and methods have been proposed (see Patent Documents 1 to 3).
In Patent Document 1, a discharge detection sensor including a probe electrode for detecting an electric potential is attached to a vacuum chamber including a processing chamber, and detects a change in the electric potential induced at the probe electrode according to the change of plasma, thereby detecting whether an abnormal discharge exists in the processing chamber. In Patent Document 2, an image of the inside of a reaction chamber is taken by a camera, and the generation and the generation position of abnormal discharge are specified by the detection of emission of light caused by the abnormal discharge. In Patent Document 3, plasma light in a plasma chamber is detected by an image sensor such as a CCD, so that a plasma process is monitored.